1. Field of the Invention
The present invention relates generally to thermally regenerative electrochemical systems. In a preferred embodiment, the present invention relates to an improved thermoelectrochemical system which utilizes phosphoric acid and ammonium phosphate as the working fluids in an acid concentration cell.
2. Description of Background Art
Thermoelectrochemical or regenerative electrochemical systems have been investigated extensively since the late 1950's. In these systems, the working substance produced in an electrochemical cell (fuel cell, battery, galvanic system, EMF cell, etc.) is regenerated by the input of thermal energy. These systems are similar to secondary batteries in many respects except that, in the former, regeneration of the electrochemically active electrode reactants is accomplished thermally in many cases rather than electrically.
Representative thermally regenerated electrochemical systems are disclosed in: 1) U.S. Pat. No. 4,292,378 issued to Krumpelt et al on Sep. 29, 1981; 2) U.S. Pat. No. 4,410,606 issued to Loutfy et al on Oct. 18, 1983; and U.S. Pat. No. 3,536,530 issued to Anthes et al on Oct. 27, 1970. The Anthes et al system includes a tellurium chloride electrochemical cell and a regeneration system for thermally regenerating the electrode reactants at temperatures of about 550.degree. C. utilizing complexing agents such as gallium chloride and aluminum chloride.
Krumpelt et al describes a thermoelectrochemical concentration cell which utilizes aluminum metal electrodes and an electrolyte composed of ethylpyridinium chloride solvent and aluminum chloride. An electrical current is generated between the electrodes by maintaining a concentration gradient such that the concentration of aluminum ions is kept low at the anode with a higher concentration being present at the cathode. The concentration gradient in the Krumpelt et al system is maintained by continually cycling the electrolyte to a still where the low boiling aluminum chloride is distilled off to provide a distillate which is high in aluminum ion concentration and a bottoms fraction which is low in aluminum ion concentration. The aluminum ion rich distillate is returned to the cathode to replenish aluminum ions plated out on the cathode while the aluminum ion poor bottoms fraction is returned to the anode to dilute the aluminum ions formed during generation of the electric current. Krumpelt et al further describes the use of iron, antimony and silicon electrodes in combination with ionic solvents such as the salts of various alkali metals, indium, ammonia and POH.sub.3 and SOH.sub.3 wherein H is a halide.
Loutfy et al discloses a thermoelectrochemical system which is based on a specific characteristic of copper in aqueous solutions. In non-complexing media, such as sulfuric acid, the redox potentials of the CU(II)/CU(I) and CU(I)/CU(O) couples exhibit an order in which the cuprous ion is less stable than the cupric ion. In certain complexing media, such as acetonitrile in sulfuric acid, the copper electrode potentials are inverted because the cuprous complex is more stable than cupric ion. Loutfy, et al utilizes this characteristic of aqueous solutions of copper to provide a variety of electrochemical cells in which electrolytes having different concentrations of complexing agent are used to generate electric potentials. In order to maintain the concentration of complexing agent within the desired ranges, the electrolytes are continually removed from the cell and thermally treated to remove at least a portion of the complexing agent from the solution.
Although the systems described above are suited for their intended purposes, there still is a continuing need to provide additional thermoelectrochemical systems which maximize the efficiencies of the thermal regeneration of reactants and of the electrochemical generation of products at high power density, and which maximize the power density itself, as well as the overall efficiency of the system. Many of the known systems tend to include complicated pumping, plumbing, and separation systems which increase the cost of the system and decrease the overall system efficiency. Further, the electrolytes presently used contain complex reactant mixtures requiring complexing agents and close control of reactant concentrations. Accordingly, there is a continuing need to provide thermoelectrochemical systems having simplified electrolyte compositions and simplified thermal regeneration systems while still providing adequate system efficiency. In addition, it would be desirable to provide a thermoelectrochemical system which utilizes conventional electrolyte materials which are readily available at low cost and present no serious environmental hazards.